They’re generally pretty flexible as long as you’re not offering really silly money for it.
You can get the Glass’ Guide price for the car here:
http://www.vauxhall.co.uk/vaux/pages/financeAndServices/valueYourCar/index.jsp
Offer the lowest of the p/x figures it comes up with. Be prepared to go up to one of the higher ones if they counter-offer and you really want the car.
Bear in mind that if you hand it back, they’ll have to transport it to an auction somewhere, and the flood of 3-year-old cars that people are handing back (PCP agreements with final values higher than they’re actually worth) means lots of stuff is going very cheaply indeed. They’ll be incurring costs all along the way so should be quite receptive to an offer that saves them all that.